1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Thevenin voltage help

  1. Dec 3, 2005 #1
    I'm trying to find the current and voltage across R2 using thenenin.
    http://img209.imageshack.us/img209/4606/thv15ph.png [Broken]
    I've managed to find Rth, which is equal to (R1 + R4)||(R3 + R5). That comes out to about 2.9 ohms. I need help with finding the Vth (Vab) though.
    http://img208.imageshack.us/img208/5002/thv28nc.png [Broken]
    Can someone please help me. What are the tricks involved here?
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Dec 3, 2005 #2


    User Avatar
    Homework Helper

    Your second picture IS the trick ... with I_2 = 0, what is V_ab ?
    That is, with I_2 = 0 , all the current thru R_1 goes thru R_3 , R_4 , and R_5 .
  4. Dec 4, 2005 #3
    Ok then so with R2 removed, the circuit is now series. That makes the current the same for the remaining resistors.

    So if I_2 is 0, then the voltage between that point is also zero? That doesnt seem right though. I still dont get it, please give me another hint.
  5. Dec 6, 2005 #4
    Can no one help.

    Would I have to use the voltage divider? How do i use it?
    Last edited: Dec 6, 2005
  6. Dec 6, 2005 #5


    User Avatar
    Gold Member

    Yes you could use the voltage divider formula. With R2 removed, you can think of [tex]V_{AB}[/tex] as the voltage drops across [tex]R_3 and R_5[/tex]

    Look at it in this way. When you remove [tex]R_2[/tex], the circuit now becomes a series circuit. Look at where point A and B are. If you where to take a voltmeter and put the probes across points A and B. Your voltmeter would be giving you to combined voltage of R3 and R5.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook